Planets abound A new study suggests each star in the Milky Way galaxy is most likely orbited by a planet - and there's a good chance that planet is closer in size to Earth, than to Jupiter.

The findings, which appear in the journal Nature, mean extrasolar planets (exoplanets) are far more numerous than previously thought, increasing the possibility of life existing beyond Earth.

"Planets orbiting stars in our galaxy is the rule, rather than the exception," write the team of researchers led by Dr Arnaud Cassan of the Institut d'Astrophysique de Paris.

Cassan and colleagues analysed data collected between 2002 and 2007 using gravitational microlensing to search for planets. This technique measures light from a distant star that is bent and amplified by the gravity of a closer object such as a star, acting like a magnifying glass. These events can be used to indicate the presence of a planet orbiting around the star nearest.

They say this technique is useful for detecting planets orbiting stars between 0.5 and 10 times the Sun-Earth distance, allowing astronomers to find planets orbiting in a star's habitable zone where water can exist as a liquid, an essential prerequisite for life as we know it.

Their analysis suggests that around 62 per cent of stars have planets similar to Earth, about 17 per cent host Jupiter-mass planets and 52 per cent have planets about the size of Neptune.

They also determined that a typical solar system has approximately four planets orbiting a star in the terrestrial zone, which is the distance from the star where you can find solid planets, such as Mercury, Venus, Earth and Mars.

And they found on average, there are 1.6 planets orbiting a star corresponding to the area bound by the orbits of Venus and Saturn.

Cassan and colleagues found the microlensing results complement existing transit and radial velocity measurements which have discovered about 1000 exoplanets so far.

The radial velocity technique detects the slight wobble of a star caused by the gravitational pull of an orbiting planet. The transit method measures tiny periodic dips in the brightness of a star as a planet orbits in front of it, as seen from Earth.

Planetary scientist Dr Simon O'Toole from the Australian Astronomical Observatory says, "The findings mean the chances of life appearing elsewhere in the galaxy go up astronomically".

"There are 200 billion stars in the Milky Way, if you have a planet on average orbiting each star, it means the chances of life existing is amazingly high," says O'Toole.

"Of course just because a star has a planet doesn't mean it has life. And just because a planet's in the habitable zone doesn't mean it's habitable."

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